1. Field of the Invention
This invention relates generally to photoelectric detectors, and more particularly to photodiode array assemblies used as detectors in spectrophotometers.
2. Description of the Prior Art
Spectrophotometers are instruments used to determine the chemical make-up of a sample by analyzing the spectral intensities of light directed through the sample. The light is divided into its component parts by the spectrophotometer, which then makes photometric measurements to determine which spectral components of the light source have been absorbed by the sample.
The light can be divided into its component parts by using a variety of devices, such as prisms, diffraction gratings, holograms, etc. The divided light tends to separate into spatially separated spectral bands which are arranged in the order of their frequencies. By analyzing the relative intensities of the spectral bands, the chemical nature of the sample can be deduced.
The intensity of the spectral bands can be measured by moving a photoelectric detector sequentially through the spectral bands, or by causing the spectral bands to move across a fixed detector. However, such methods are slow and requires precision transport mechanisms. A more desirable detector for use in spectrophotometers is a photodiode array comprising several hundred individual photodiodes formed on a common, semiconductor substrate. Each of the photodiodes of the array can correspond to a particular spectral band, allowing the intensities of all of the spectral bands to be measured simultaneously.
A problem with prior art photodiode array spectrophotometers is that a photodiode of the array can be overloaded if a particularly intense spectral band falls upon it. For example, it has been found that the light emitted by a deuterium discharge lamp source has a strong spectral line at 656 nanometers (nm) which can overload the photodiode corresponding to that wavelength and thus the analog-to-digital (A/D) converter which couples the photodiode array to the remainder of the system.
The aforementioned problem has been addressed by the Hewlett-Packard Company of Palo Alto, California with their models HP 8451 and HP 8452 diode array spectrophotometers. The solution with both the HP 8451 and HP 8452 is to apply an opaque mask over the photodiode array to partially shade the photodiodes corresponding to the strong spectral bands.
As will be discussed in greater detail subsequently, the prior art photodiode array masks tend to introduce aberrations in the absorption curve generated by the spectrophotometer. These aberrations have been a persistent problem in the industry. For example, the Hewlett-Packard Company has been shipping spectrophotometers using the prior art photodiode array masks since approximately 1979, and has made continuing efforts to solve the problem of the aberrations caused by the prior art masks. Despite major research efforts by recognized experts in the field of spectrophotometry, the solution to this problem heretobefore eluded discovery.